Ink-jet printing is a non-impact printing process in which droplets of ink are deposited on a print medium in a particular order to form alphanumeric characters, area-fills, and other patterns thereon. Low cost and high quality of the hardcopy output, combined with relatively noise-free operation, have made ink-jet printers a popular alternative to other types of printers used with computers.
The non-impact printing process of ink-jet printing involves the ejection of fine droplets of ink onto a print medium such as paper, transparency film, or textiles in response to electrical signals generated by a microprocessor. There are two basic means currently available for achieving ink droplet ejection in ink-jet printing: thermally and piezoelectrically. In thermal ink-jet printing, the energy for drop ejection is generated by electrically heated resistor elements, which heat up rapidly in response to electrical signals from a microprocessor to create a vapor bubble, resulting in the expulsion of ink through nozzles associated with the resistor elements. In piezoelectric ink-jet printing, the ink droplets are ejected due to the vibrations of piezoelectric crystals, again, in response to electrical signals generated by the microprocessor.
Commercially-available thermal ink-jet printers, such as DeskJet.RTM. printers available from Hewlett-Packard Company, use inks of differing hues, namely, magenta, yellow, and cyan, and optionally black. The particular set of colorants, e.g., dyes, used to make the inks is called a "primary dye set." A spectrum of colors, e.g., secondary colors, can be generated using different combinations of the primary dye set.
Any given perceived color can be described using any one of the color spaces, such as CIELAB, and Munsell, as is well known in the art. For example, in the Munsell color space a given color is defined using three terms, Hue, Value, and Chroma. Similarly, in the CIELAB color space, a color is defined using three terms L*, a*, and b*. L* defines the lightness of a color, and it ranges from zero (black) to 100 (white). The terms a* and b*, together, define the hue and chroma characteristics of a given color. The term a* ranges from a more negative number, green, to a more positive number, red. The term b* ranges from a more negative number, blue, to a more positive number, yellow. Additional terms such as hue angle (h.degree.) and chroma (C*) are used to further describe a given color, wherein EQU h.degree.=tan.sup.-1 (b*/a*) Equation 1 EQU C*=(a*.sup.2 +b*.sup.2).sup.1/2 Equation 2
In the CIELAB color space, delta E (.DELTA.E) defines the difference between two colors, such as the color of the printed image as initially printed and the color after exposure to light (i.e., lightfade)--the higher the .DELTA.E number, the more difference between the two colors. EQU .DELTA.E=[(L*.sub.2 -L*.sub.1).sup.2 +(a*.sub.2 -a*.sub.1).sup.2 +(b*.sub.2 -b*.sub.1).sup.2 ].sup.1/2 Equation 3
.DELTA.E=(.DELTA.L*.sup.2 +.DELTA.a*.sup.2 +.DELTA.b*.sup.2).sup.1/2 Equation 4
In general, a successful ink for color ink-jet printing must have the following properties: good crusting resistance, good stability, the proper viscosity, the proper surface tension, good color-to-color bleed alleviation, rapid dry time, no negative reaction with the vehicle, consumer-safety, and low strike-through. When placed into a thermal ink-jet system, the ink set must also be kogation-resistant.
In addition, the inks must be able to provide printed images having good color characteristics, such as correct hue and high chroma. While formation of colors on plain paper is required, it is also necessary that the inks be useful on other print media, such as transparency film and coated paper. Another requirement for the ink set is to provide a hard copy output that is lightfast, thus preserving the integrity of the original color information.
While some of these conditions may be met by ink vehicle design, other conditions must be met by the proper selection and combination of the colorants. The selection of the colorants becomes especially important when additional limitations are placed on the choice of the colorants because of other system requirements, such as the color-to-color bleed control mechanism.
Regardless of whether an ink is dye-based or pigment-based, ink-jet inks commonly face the challenge of color-to-color or black-to-color bleed control. The term "bleed," as used herein, is defined to be the invasion of one color into another, once the ink is deposited on the print medium, as evidenced by a ragged border therebetween. Bleed occurs as colors mix both on the surface of the paper substrate as well as within the substrate itself. The occurrence of bleed is especially problematic between a black ink and an adjacently-printed color ink because it is all the more visible. Hence, to achieve good print quality, bleed must be substantially reduced or eliminated such that borders between colors are clean and free from the invasion of one color into the other.
One approach used for controlling bleed between the printed images, as disclosed in U.S. Pat. No. 5,428,383 entitled "Method and apparatus for preventing color bleed in a multi-ink printing system," issued to Shields et. al., is to provide a first ink composition comprising a first colorant; and a second ink composition comprising a second colorant and a precipitating agent (e.g., inorganic salts) which will react with the first colorant in the first ink composition so that upon contact of the first ink and the second ink on the printing medium a precipitate is formed, thus preventing color bleed between the first ink composition and the second ink composition.
To take advantage of the precipitation mechanism for controlling bleed, it becomes important to have inks that can provide all the above performance requirements while maintaining reliability in the presence of precipitating agents.
Therefore, a need exists for inks that can provide the appropriate lightfastness and reliability in an environment having a relatively high concentration of precipitating agents.